Microbial iron transport and mobilization is achieved using very powerful low-molecular-weight chelating agents which solubilize ferric ion by coordination to form octahedral complexes of high-spin Fe 3 ion. These compounds, called siderophores or siderochromes, are manufactured by microorganisms as needed to extract iron from their solution environment and concentrate it within the organism. Iron is essential for microbial growth and the iron transport process has been linked to the pathogenicity of several microbial infections. Transport of the iron occurs via several different mechanisms. Study of this transport is complicated by the kinetic lability of high-spin Fe(III) complexes in aqueous solution. Replacement of Fe(III) by Cr(III) gives kinetically inert complexes that can be used in transport studies with living cells. The Cr(III) complexes can also be used to determine the coordination geometry of the siderophore complexes using vis-uv and CD spectroscopy. The thermodynamic properties (formation constants and redox behavior) and reaction kinetics (iron exchange and release rates) of the siderophores are being characterized to delimit the possible in vivo behavior of these iron transport agents. Structure analyses, including X-ray diffraction, of the siderophores and their metal complexes are being used to characterize the free and metal-bound ligand geometries.